Neutralizing ion chamber charging currents



Oct. 9, 1956 J. W. RALLS NEUTRALIZING ION CHAMBER CHARGING CURRENTS Filed July 51, 1955 United States Patent() NEUTRALIZING N CHAMBER vCHARGING.

"cURREN'rs John W. Ralls, Schenectady, N. Y., assignor, by mesne assignments, V,to the United States of America as yrepresented vby thefSecretary of the Navy Application July 31, 1953,*"seri1Nt-1. '371,581

Y4 Claims. (erase-83.69

'This invention relates. to a 4device -for detectingand measuring certain substances present in gases and vapors and particularly relates 4"to t'lrat'type vof device wherein tan "ion chamber comprisingthreecoaiial electrodes is used.

ln the type of device with which this invention is concerned, a high voltage is applied to one of the electrodes of the ion chamber to collect ions present in the air stream, these ions causing a current to be developed in an electrometer circuit wherein the current is measured. However, the drift-stability requirements of the high voltage D.C. supply are quite severe. Any drift in the value of the high voltage supplied produces charging current in the electrometer circuit which the electrometer cannot distinguish from the signal current produced by the collection of ions. The value of this charging current is given by the equation:

Assume that the charging current through the electrometer is to be held to a value not exceeding the smallest signal current to be measured, 10w13 amperes for example, and that C has a typical value of 200 micro-microfarads. Then dV/dt, the high voltage drift rate, must not exceed 0.0005 volts/ second. A typical value of high voltage used is 2500 volts. This means that the drift rate of the high voltage supply should never exceed, even for short periods, 0.00002% per second.

It is, therefore, the principal object of this invention to provide a drift-neutralization circuit to reduce the power supply stability requirement.

Another object of this invention is to provide a driftneutralization circuit which is relatively simple and easy to install and maintain.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a schematic diagram of the coaxial electrodes and a general representation of the ordinary electrical system used therewith.

Fig. 2 is a schematic diagram illustrating an electrical system embodying this invention.

Referring now in greater detail to the drawing wherein similar reference characters refer to similar parts, there is shown in Fig. 1 an ion chamber 10 comprising an outer cylindrical electrode 12, an intermediate, coaxially arranged, cylindrical electrode 14 and an inner cylindrical electrode 16. The outer electrode 12 is connected to ground at 18 and high voltage is applied to intermediate electrode 14 from a high voltage source 20. The inner electrode 16 is connected to ground through a high resistance (e. g. 1011 ohms) electrometer circuit 22. An air stream, driven by a blower, not shown, passes between electrodes 14 and 16. Atmospheric ions, of polarity determined by the high voltage polarity, are collected by electrode 16 and produce a current through the elec- 2,766,386 Patented Oct. 9, c1956 ICC 2 trometer circuit which'is v:proportional to vthe v.number lof ions collected, and hence the number 'of ions -of the chosen polarity present in the air stream enteringthe chamber. The current to be measured by the electrometer circuit may be as small as 10-1l amperes.

As observed above, however, any drift in the value of the applied high voltage produces charging current through the electrometer circuit which is indistinguishable from thesignal current. To reduce thispower supply stability requirement, the system illustrated in Fig. `2 ha's been devised. The system includes a high voltage supply source v24 and an electrometer circuit r25 which `are connected to the capacitors l'26, 28 and 30 -and the resistors l32 and 34. Thesercap'acitors Vandresistors'trepresent the equivalent circuit f'o'f the 'ion chamber, -tli'e capacitors representing Ithe inter-electrode capacitan'c'esfof YAthe :ion .chamber and the resistors representing thelleak- Usually R32 is much larger than R42, and C38 is much larger than C26. The balance conditions then simplify to:

Capacitor 40 and resistor 44 may be adjustable to provide a good balance.

The neutralization circuit reduces the high voltage applied -to the ion chamber by the factor which is equal to the factor C36 C30-F036 This reduction may be made small by making C36 the collection of ions. The value of this charging curper capacitor has been used for C36 which is twentyve times as large as C30 so that 96% of the high volt- .age from the supply is usefully applied to the ion chamber. The leakage resistance of the Fast capacitor is negligibly large. As an example, C38 may be l microfarad, C40 may be 25 microfarads, R42 may be 5 megohms and R44 may be 0.2 megohm. Since only 4% of the high voltage is applied across the capacitor 40, it may be of the low-voltage variety.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed as the invention is:

l. Apparatus for detecting a foreign substance in an atmosphere comprising an ion chamber consisting of coaxially arranged inner, intermediate and outer cylindrical electrodes; a high voltage source having one terminal connected to the intermediate electrode of the charnber; an electrometer circuit electrically connected between the inner and the outer electrodes of the chamber; a parallel resistor capacitor circuit electrically connected between the intermediate and outer electrodes of the chamber; a parallel resistor capacitor circuit electrically connected between the outer electrode of the chamber and the other terminal of the high voltage source; and a condenser electrically connected between the said other terminal of the high voltage source with the inner electrode of the chamber.

2. The apparatus of claim 1 wherein the ratio of the impedances of the parallel resistor capacitor circuits is substantially equal to the ratio of the capacitance the said condenser and the interelectrode capacitance between the intermediate and inner electrodes of the chamber.

3. An electrical network comprising a high voltage supply source having a pair of output terminals; a parallel resistance capacitance circuit connected between one of the said terminals of the supply source and a first cylindrical electrode of an ion chamber; another parallel resistance capacitance circuit connected between the other of the said terminals of the supply source and the said first cylindrical electrode of the chamber; a capacitor element connected between the said other terminal of the supply source and a second cylindrical electrode of the ion chamber, the second electrode of the chamber being coaxially arranged with respect to the first electrode; means for electrically connecting the said one terminal of the supply source to a third cylindrical ele'ctrode of the ion chamber, the third electrode of the chamber being coaxially arranged with respect to the said rst and the second electrodes; and an electrometer circuit connected between the said rst and the second electrodes.

4. The electrical network of claim 3 wherein the ratio of the impedances of the said parallel resistance capacitance circuits is substantially equal to the ratio of the capacitance of the said capacitor element and the interelectrode capacitance between the said second and third electrodes of the ion chamber.

FOREIGN PATENTS Great Britain Feb. 1, 1933 

